Field
The present invention relates to a manufacturing method of microelectromechanical (MEMS) devices and especially to manufacturing a micromechanical device layer as defined herein. This application claims foreign priority to Finland Patent Application Number 20155352, filed on 15 May 2015, entitled “A multi-level micromechanical structure and a manufacturing method thereof,” which is hereby incorporated by reference in its entirety.
Description of the Related Art
A Micro-Electro-Mechanical System (MEMS) device has moving elements under control of integrated microelectronics and contains micro-circuitry on a tiny silicon chip into which some mechanical device, such as a micro sensor, and a micro actuator has been manufactured. These micro sensors and micro actuators constitute functional elements of micro-electromechanical (MEMS) devices. The physical dimensions of MEMS devices can vary from below one micron to several millimeters.
MEMS devices convert a measured mechanical signal into an electrical signal. MEMS sensors measure a mechanical phenomenon and the related electronics then process the information derived from the sensor elements and through some decision making capability direct the actuators to respond by e.g. moving, positioning, or regulating in order to thereby control the environment for some desired outcome or purpose. MEMS devices can thus comprise both drive elements and sensing elements to perform specific functions.
Examples of systems fabricated using MEMS technology are pressure sensors, accelerators for measuring acceleration of moving objects, micropositioners, such as micromirrors, optical switches or scanners, and gyroscopes for measuring angular velocity of rotating objects.
MEMS devices might be capacitive or make use of piezoelectric transduction.
A key element in a capacitive MEMS device is a variable capacitor formed between two electrodes. These may comprise a stationary electrode and a movable electrode attached to a suspended proof mass. Alternatively the device may comprise two movable electrodes. An inertial mass, and the movable electrode attached to it deflects in response to acceleration in an accelerator or Coriolis force exerted on the inertial mass when an angular velocity is applied to a gyroscope and used for measuring this velocity. The amount of deflection can be sensed from changes in capacitance from the changes in the gap between the two electrodes due to deflection. The change in capacitance can be produced by the change in overlapping area between the fixed and movable electrodes as well. In case of vertical movement the actuation and sensing is typically realized by comb designs, where one comb pair is fixed to the substrate and other comb pair is able to move on Z-direction through a spring structure. Moreover, when the fixed and movable combs have offset in vertical direction, the direction of movement can be measured in a linear manner by various design and measurement arrangement.
Accelerometers are acceleration sensors. An inertial mass suspended by springs is acted upon by acceleration forces that cause the inertial mass to be deflected from its initial position. This deflection is converted to an electrical signal, which appears at the sensor output.
An accelerometer comprises an inertial mass, one side of which is fixed to a carrier while the other is suspended. It further comprises means for detecting the movement of the membrane under the effect of acceleration. This constitutes a sensor, which senses acceleration force.
Inertial sensors are a type of accelerometer and are one of the principal commercial products that utilize surface micro machining.
When things rotate around an axis they have angular velocity. Gyroscopes, or gyros, are devices that measure or maintain rotational motion. In MEMS devices, vibration is typically used as primary motion of the gyroscope. In a vibrating sensor of angular velocity, for example a gyroscope, a certain known primary motion is induced and maintained in the sensor. When a mass is vibrating in one direction called the primary motion and rotational angular velocity is applied, the mass experiences a force in orthogonal direction as a result of the Coriolis force. Resulting physical displacement caused by the Coriolis force may be then read from, for example, a capacitive, piezoelectrical or piezoresistive sensing structure.
When MEMS technology is used to implement gyroscopes, these have an inertial structure suspended above a substrate and associated electronics that both senses movement of the suspended inertial structure and deliver the electrical signal caused by the sensed movement to an external computer. The computer processes the sensed data to calculate the property being measured.
Structures for vibrating gyroscopes are formed e.g. by etching a semiconductor wafer to form a proof mass used as a reference in the measurement. The proof mass is suspended by a spring system, such as elastic beams, to a substrate. An electronic drive circuit which may be on the same substrate applies an alternating drive current to driving electrodes which vibrate the proof mass in a drive direction. The electrical drive mechanism vibrates the proof mass along a drive axis and the electrodes build a capacitance together with the proof mass for detecting motion of the proof mass along a sense axis perpendicular to the drive axis. A triple axis MEMS gyroscope can measure rotation around three mutually orthogonal axes, while single and dual axis gyros measure the rotation around one or two of these axes.
Comb structures may be utilized in sensor devices for detection and/or driving purposes. Comb structures may also be used as actuators, a.k.a. comb drivers in a micromechanical device for various purposes. A comb structure typically includes at least one stator, in other words a non-moving part and a rotor, in other words a moving part. Typically, stator(s) and rotor(s) of a comb structure include two or more so called comb fingers, which can be described as longitudinal extensions of the structure, which may gave varying shape, length and width. A reason for including such fingers in a comb structure is for increasing surface area of electrodes mutually affecting each other, enabling for instance greater forces between electrodes. A voltage difference applied to a comb structure may drive micro motors and micro positioning devices, such as micro mirrors, optical switches, optical guides, scanners, laser printers, wavelength-selective switches, diffraction gratings and so on as known by a person skilled in the art.
Term die refers to one small block of the semiconducting material, on which a given functional circuit, a chip, is fabricated. In the manufacturing of the micro-electronic devices, each individual die contains one of the integrated circuits. During manufacturing, a wafer with up to thousands of circuits is cut into rectangular pieces, each called a die. The integrated circuits are produced in large batches on a single wafer.
Etching is a critically important process module, and every wafer undergoes many etching steps before it is complete. For many etch steps, part of the wafer is protected from the etchant by a “masking” material which resists etching. The masking material is e.g. a photoresist which has been patterned using photolithography. The patterning shows which parts of the wafer should be etched.
In anisotropic etching, the etching rate is different in horizontal and vertical direction.
US2006/0284514 presents an actuator having a vertical comb electrode structure. This actuator is manufactured in a two side etching process, which leads to having insulation layer in the fixed comb electrodes.
U.S. Pat. No. 7,088,030 presents a HARM structure where the formed comb structures have lateral strengthening structure formed at vertically peripheral walls.
A problem relating to this prior art is that while the comb structures are not formed of homogenous material, the accuracy of the comb electrodes is not optimal. While different materials have different electron affinity, the non-homogenous materials have electrostatic differences and some electronic biasing arrangements may be required for the comb structure in order to compensate any unwanted potential differences between the comb fingers caused by the materials. Similar problem arises even if different parts or portions of the die material in the comb structures is processed with different processing methods causing the remaining electrodes having different or varying surface structure.
EP1798195 presents a comb-type electrode structure where two electrodes have been manufactured from two different substrates. Although the two substrates may be of same material and processed by similar process, combining two separately created layers of die always induces problems with alignment of the two layers, which has to be taken into account in the design of the electrodes: required tolerances are high which leads to bulky designs and ineffective use of the die area. Any misalignment occurring in placement of the layers will cause decreased accuracy for the comb electrode.